Our goal is the development of cell separation methods for the specific isolation of immune cells, particularly for varieties of antigen-reactive cells (ARC) involved in cellular immune reactions, and for their subcellular fractionation in order to study the mechanisms involved in the development of immune reactivities and immune macromolecules. Populations of cells containing ARC are tested for binding to the cell surface antigens of target cells attached to insoluble supports. Separated populations are tested for cytotoxic effector cells (CTL) and their precursors, for activity in allograft rejection and graft-versus host reaction and in the mixed lymphocyte reaction. T cell subpopulations from thymus and spleen are also separated by and characterized with specific reagents such as peanut aggulutin and antibodies to the Lyt and CTL differentiation antigens. Surface molecules of target cells are isolated to test their binding to ARC.